Ceramic Humidity Sensors Microstructure and Simulation of Humidity Sensitive Characteristics

Abstract

The relationship between the microstructure and the humidity sensitive characteristic was studied for MaAl₂O₄ and MgFe₂O₄ elements. The microstructure of these elements was controlled by selecting starting materials and sintering conditions. The humidity sensitivity of the element for low relative humidity was enhanced with an increase in surface area and pore volume, expecially below 300Å in diameter. MaAl₂O₄ element, prepared by metal alkoxides and sintered at 1000°C for 5 h, exhibited a high humidity sensitivity. However, the response time was longer because the adsorption or desorption rate of water was controlled by the diffusion rate of water vapor through the micropores. As for MgFe₂O₄ elements, having a small surface area and a small pore volume, the impedance changes to the relative humidity changes were smaller than that of MaAl₂O₄ elements. The humidity sensitive characteristics of MgFe₂O₄ elements were influenced by both the microstructure and the impedance of the element at 0%1Z1-1. A theoretical calculation of the humidity sensitive characteristics was adapted for these element to clarify the relationship between the microstructure and the humidity sensitivity. In this calculation, all pores of the element were assumed to be filled with condensed water according to the Kelvin equation. The impedance of the element at a certain relative humidity was calculated by using the impedance of the element at 0%ZH, the porosity, the total pore size distribution, the conductivity of condensed water, electrode area and the thickness of the element. From the results obtained from the theoretical calculations, the conductivity of condensed water was found to be larger than 8.20×10^-7 S⋅cm^-1, which was an estimated value for the water exposed to the air containing 300 ppm of CO₂ at 20°C. This was confirmed by the conductivity enhancement of water after a boiling treatment of the specimen. Although the surface area and pore volume became larger after the boiling treatment, the impedance changes of the elements to relative humidity became smaller. This fact indicated that most of the dissolved ions existed on the surface of the element were washed out by this treatment. The theoretical calculation was also adapted for the elements after boiling treatment and the calculated values agreed approximately with experimental ones. Therefore, it was concluded that the humidity sensitive characteristics could be estimated by the theoretical calculation based on the pore size distribution.